Diode-lasers are efficient devices for converting electrical power into coherent optical power. An edge-emitting diode-laser has a diode-laser emitter, which is waveguide resonator, grown on a single-crystal substrate. The diode-laser emitter emits laser-radiation through an end facet in an emission direction. The diode-laser emitter is typically between 500 micrometers (μm) and 2,000 μm long, between 40 μm and 200 μm wide, and about 1 μm high. The laser-radiation is weakly divergent in a slow-axis direction (parallel with emitter width) and strongly divergent in a fast-axis direction (parallel with emitter height). The slow-axis, fast-axis, and emission directions are mutually perpendicular.
For high-power applications, a diode-laser bar having a plurality of diode-laser emitters provide a convenient way to scale the optical power available from a single diode-laser emitter. A diode-laser bar has typically between 10 and 60 such diode-laser emitters spaced apart and arranged in a “horizontal” linear array thereof. The distance between the centers of adjacent diode-laser emitters is generally referred to as “pitch”. A typical diode-laser bar has pitch between 150 μm and 500 μm. Diode-laser emitters are usually on a “top” surface of the diode-laser bar, which is typically between about 50 μm and 200 μm in height. Laser-radiation is emitted from all the diode-laser emitters through a common “front facet” in a common emission direction.
A “packaged” diode-laser bar is mounted on a cooling base for mechanical protection and to remove waste heat, usually by soldering. However, soldering a relatively-thin diode-laser bar onto a rigid cooling base causes mechanical stress that can displace the diode-laser emitters from an ideal linear alignment. The displacement from the ideal linear alignment is referred to as “smile” by practitioners of the art.
Packaged diode-laser bars may be selected for minimal smile. However, additional metrology and reduced yield increase cost. Smile may be mitigated by selecting a ductile “soft solder”, such as indium. However, soft solders are more vulnerable to failure, due to atomic diffusion and oxidation. Various schemes have been proposed to pre-compensate the mechanical stress caused by “hard solder”, such as gold-tin. However, such schemes add cost and complexity to packaging.
Lasers have become essential sources for uniform illumination in a wide range of applications, including surface inspection of semi-conductor materials, thermal annealing of display-screen glass, and rapid assay of bio-medical fluids. A common requirement is an elongated beam of laser-radiation that uniformly illuminates a line on a flat surface or a plane in a volume of transparent material. Such elongated beams of laser-radiation are referred to generally as “line-beams.” Diode-laser bars have advantages as line-beam sources, including high-power and an elongated emission cross-section. However, it is necessary to overcome an inherent non-uniformity of emission from a plurality of spatially-distributed diode-laser emitters.
An optical device for transforming a beam of radiation to make it uniform in power across a cross-section of the beam is generally referred to as a “beam homogenizer”. Beam homogenizers often include a “micro-lens array”, comprising a plurality of tiny lenses, each much smaller than the incident beam. Each micro-lens becomes a source contributing to the transformed beam. “Pitch” of a beam homogenizer is the distance between centers of adjacent micro-lenses.
Additional optics are required to collect and shape the plurality of beams emerging from all the micro-lenses intercepting the incident beam. A linear array of micro-lenses may be used as a beam homogenizer for an elongated beam of laser-radiation emitted by a diode-laser bar. One example of such a beam homogenizer is described in U.S. Pat. No. 7,265,908. However, line-beams created by such devices are degraded by smile. Variances in smile between diode-laser bars cause variances in the dimensions of line-beams created by beam homogenization. While prior-art devices can create a uniform line-beam at one location along a transformed beam, they are unable to provide uniform illumination along a range of locations.
There is need for an improved device for shaping a line-beam from a diode-laser bar that is insensitive to smile and to variances in smile. Preferably, the beam-shaping device creates a line-beam that remains uniform over a range of locations along the emission direction.